Support virtio-balloon deflate-on-oom flag

[valpackett]

qemu and Firecracker have an option to set VIRTIO_BALLOON_F_DEFLATE_ON_OOM e.g. which is purely a control flag for guest OOM behavior (does not actually change VMM behavior). We definitely want it in desktop scenarios (muvm) so libkrun should provide a way to toggle it..

[slp]

Right now, we only support free page reporting on virtio-balloon. This means the balloon is never inflated nor deflated, so that flag would probably not have any effect at all.

[valpackett]

Hm, right, the other events are enqueued but never processed (what actually makes sure the guest doesn't send them? o.0 I didn't think you could tell the guest to never inflate/deflate?).. I'll try to take a look at implementing it all.